The Bogibeel Bridge in Assam has a two-line railway track on the lower deck and a three-lane road on the top deck. It will reduce the Delhi to Dibrugarh train travel time by about three hours.

Assam's Bogibeel Bridge was constructed at an estimated cost of Rs 5,900 crore.

Guwahati: The foundation stone for Assam's Bogibeel Bridge may have been laid in 1997, but it is only today that the double-decker rail-and-road bridge will open to traffic. Prime Minister Narendra Modi will do the honours. Billed as India's longest railroad bridge, the five-kilometre structure spans the waters of the mighty Brahmaputra River. Bogibeel Bridge will connect Assam's Dibrugarh and Dhemaji districts, bordering Arunachal Pradesh. It will cut the travel time between the two points by four hours and help avoid a 170-km detour. With its proximity to the China border, the bridge also has tremendous significance for India's defence and has been built strong enough to support the movement of tanks and even fighter jet landings.

Here are 10 things you should know about the Bogibeel Bridge:

Designed like a bridge that links Sweden and Denmark, the Bogibeel Bridge is India's only fully-welded bridge for which European codes and welding standards were followed. A fully welded bridge has a low maintenance costs, its engineers say.

The bridge has a two-line railway track on the lower deck and a three-lane road on the top deck. It will reduce the Delhi to Dibrugarh train travel time by about three hours.

Inaugurating the bridge, PM Modi will also flag off a train - the Tinsukia-Naharlagun Intercity Express, which will run five days a week.

The 4.9 km-long bridge was constructed at an estimated cost of Rs 5,900 crore - exceeding its initial budget of about Rs 3,200 crore and originally planned length of 4.31 km.

The foundation stone of the project was laid by former Prime Minister HD Deve Gowda on January 22, 1997, while work began on April 21, 2002 under the Atal Bihari Vajpayee-led government. December 25 happens to be Vajpayee's birth anniversary.

"Early flood in the river Brahmaputra restricted the working season to a very short period of approximately five months - from November to March - and demanded huge mobilization of construction equipment," Chief Engineer Mohinder Singh told news agency PTI, explaining the delay in the project.

The project took 30 lakh bags of cement - enough to fill more than 41 Olympic swimming pools and 19,250 metres of reinforcement steel - well over twice the height of Mount Everest.

The bridge will be a massive help for residents of the remote districts of Anjaw, Changlang, Lohit, Lower Dibang Valley, Dibang Valley and Tirap in Arunachal Pradesh besides the Dibrugarh and Dhemaji districts in Assam.

The Bogibeel Bridge is likely to play an important role in defence movement along the India-China border in Arunachal Pradesh. The three road lanes of the bridge can act as three landing strips for the Air Force, news agency PTI reported.